Explain Function of Seven Layers of OSI Reference Model


Data Link






Physical Layer:

The Physical layer coordinates the functions required to transmit a bit stream over a physical medium,. It deals with the mechanical, electrical specifications of the primary connection such as cables, connections & signaling options that physically link two nodes on a network.

Signals:  What types of signals are useful for transmitting information?
Encoding:  Howare bits (0s & 1s) to be represented by available signaling system?
Topology:  How are network devices arranged? Do they pass data directly to each other or through an intermediary?
Line Configuration:  How two or more devices are linked physically? Is the line available or not?
Data Transmission Mode:  It does transmission flow one way or both ways between two connected devices? Or does it alternate?
Medium:  What is the physical environment for the transmission of data?

Delivery of data units in the data link layer are given below:
Error Handling:  Data link layer protocols provide for data recovery, usually by having the entire frame re-transmitted.
Access Control:  When two or more devices are connected to the same link, the data link layer protocols are necessary to determine which device has control over the link at any given time.
Flow Control:  To avoid overwhelming the receiver, the data link layer regulates the amount of data that can be transmitted at one time. It adds identifying numbers to enable the receiving mode to control the ordering of the frames.
Node to Node Delivery: The data link layer is responsible for node to node delivery.
Addressing:  Headers and Trailers added at this layer include the physical addresses of the most recent node and the next intended node.
Packetizing:  Each LAN or WAN has its own type of packet. Therefore packet coming from network layer is inserted inside a frame with a new label.

Network Layer:

Network layer is responsible for the source to destination delivery of a packet across multiple network links. Specific responsibilities of the network layer include of the following:

Source to destination delivery:  Moving a packet from its point of origin to its intended destination across multiple network links.
Address Transformation:  Interpreting logical addresses to find their physical equivalents.
Multiplexing:  Using a single physical line to carry data between many devices at the same time.
Logical Addressing:  Inclusion of the source and destination addresses in the header.
Routing:  Deciding which of multiple paths a packet should take.

Transport Layer:

The transport layer is responsible for source to destination (end to end) delivery of the entire message.

End to End Message Delivery:  Overseeing the transmission and arrival of all packets of a message at the destination point.
Service Point (port) Addressing:  Guaranteed delivery of a message to the  appropriate application on a computer running multiple applications.
Segmentation and reassembly:  Dividing a message into transmittable segments, and marking each segment with a sequence number.
Connection Control:  Deciding whether or not to send all packets by a single path.
Reliability: Damage controls, loss control, order control, duplicacy control of packets.

Session Layer:

The session layer is the network dialog controller. It establishes, maintains, and synchronizes the interaction between communication devices. The session layer validates and establishes connections between users.

 Session Management:  Dividing a session into sub sessions by the introduction of checkpoints and separating long messages into shorter units, called dialog units appropriate for transmission.
Synchronization:  Deciding in what order to pass the dialog units to the transport layer, and where in the transmission to require confirmation from the receiver.
Dialog Control:  Deciding who sends, and when.
Graceful Close:   Ensuring that the exchange has been completed appropriately before the session closes.

Presentation Layer:

The presentation layer ensures interoperability among communicating devices. Functions at this layer make it possible for two computers to communicate even if their internal representations of data differ (e.g. When one device uses one type of code and the other uses another).

Translation:  Changing the format of a message from that used by the sender into one mutually acceptable for transmission. Then, at the destination, changing that format into the one understood by the receiver.
Encryption:  Encryption and decryption of data for security purposes.
Compression:  Compressing and decompressing data to make transmission more efficient.
Security:  Validating passwords and login codes.

Application Layer:

The application layer enables the user, whether human or software, to access the network. It provides user interface and support for services such as electronic mail, remote file access and transfer, shared database management, and other types of distributed information services.

File Access, Transfer, and Management:  Allows a user at a remote computer to access files in another host (to make changes or read data); to retrieve files from a remote computer for use in the local computer, and to manage or control files in a remote computer.
Mail Services:  Provides the basis for electronic mail forwarding and storage.
Directory Services:  Provides distributed database sources and access for global information about various objects and services.

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